Add checkboxes action payload

[seratch]

Summary

Related to slackapi/node-slack-sdk#964, this pull request adds a type for checkboxes-typed payload.

There are a few related tasks:

• make a pull request to v2 (@slack/node@next) branch [v2] Add checkboxes action payload #409
• add the block elements to @slack/types - Fix #964 Add types for checkboxes node-slack-sdk#966

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #408 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #408   +/-   ##
=======================================
  Coverage   81.33%   81.33%           
=======================================
  Files           7        7           
  Lines         525      525           
  Branches      150      150           
=======================================
  Hits          427      427           
  Misses         70       70           
  Partials       28       28

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